1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium such as a magnetic tape and a magnetic disc comprising a hexagonal system barium ferrite powder as a recording element.
2. Description of the Related Art
Requirement for high density recording of a magnetic recording medium increases year by year. A horizontal recording medium in which acicular magnetic powder is oriented in a magnetic layer has such a disadvantage that rotational demagnetization significantly increases as the recording density is increased. Recently, therefore, a vertical magnetic recording medium in which signals are recorded in a vertical direction to a magnetic layer is being developed.
As a magnetic layer formed in such a vertical magnetic recording medium, a layer which is formed by coating platelet particles of hexagonal system barium ferrite powder, with orienting the platelet surfaces in parallel to a medium surface is considered to have better properties, such as productivity and durability, than a thin metal layer consisting of magnetic metal, such as Co--Cr.
However, a conventional magnetic recording medium using the hexagonal system barium ferrite powder has a fatal disadvantage in that it has unsatisfactory storage properties since an output of recorded signals attenuates, namely so-called "demagnetization by cooling" occurs, when the recording medium is once subjected to a low temperature after signals are recorded, and then reproduced at room temperature.
In the course of research for overcoming the above disadvantage, it was found that since a coercive force of the magnetic recording medium comprising the hexagonal system ferrite powder as the recording element largely varies with temperature and usually has such temperature dependency that the coercive force is low at low temperatures and high at high temperatures, when the recording medium is kept at low temperatures after recording, the coercive force is decreased so that the output of the signals deteriorates.